Changes in variable cam timing (VCT) affect engine volumetric efficiency. Typical engine control methods use volumetric efficiency characterization, calibrated off-line at specific engine conditions, to perform on-line computations for functions that require such information. For example, in some control methods, volumetric efficiency information and intake manifold pressure measurements are used to compute engine air flow. Further, some control methods use volumetric efficiency to compute estimated intake manifold pressure from engine air flow values.
However, errors in cam angle measurement due to engine build variation or other sources can introduce errors in the estimated volumetric efficiency, and these errors propagate through air flow and intake manifold pressure estimations. Moreover, aggressive use of VCT systems for either late exhaust valve opening or late intake valve closing (LIVC or Miller-cycle in boosted engines) makes volumetric efficiency very sensitive to engine build variation.
A common method to correct for some engine build variation in cam timing is to ensure that the measured cam angle relative to some physical end-of-travel position is zero when the cam is assumed to be in that position, for example, the unpowered, default position. Such a method corrects for some sources of engine build variation, but not all. For example, misalignment of the physical end-of-travel position with respect to physical valve opening or closing events is not corrected.
The inventors herein have identified the above issues and devised several approaches to address it. In particular, methods and systems for correcting cam angle measurements for engine-to-engine build variation are disclosed. In one example, a method comprises learning cam angle corrections to update a measured cam angle responsive to air-fuel ratio errors during selected conditions, and learning air and fueling errors responsive to the air-fuel ratio error otherwise. In this way, cam angle errors due to engine build variation may be corrected, thereby improving other air and fuel adaptation methods and improving engine emissions.
In another example, a method comprises generating a first air-fuel ratio estimate based on engine operating conditions, generating a second air-fuel ratio estimate based on modified engine operating conditions, generating a first error based on the first air-fuel ratio estimate and a measured air-fuel ratio, generating a second error based on the second air-fuel ratio estimate and the first air-fuel ratio estimate, generating a cam angle correction based on the first error and the second error, and updating a cam angle measurement based on the cam angle correction. In this way, off-line volumetric efficiency characterization information may be utilized to isolate a cam timing contribution to air-fuel ratio errors.
In another example, a system for controlling an engine comprises a controller configured with instructions stored in non-transitory memory, that when executed, cause the controller to learn cam angle corrections responsive to air-fuel ratio errors during selected conditions. In this way, a vehicle engine can eliminate variable cam timing calibration errors specific to the engine.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.